feat(minifier): inline single use variables within the same variable declarations

[sapphi-red]

Compress

var a = foo, b = a;

into

var b = foo;

refs #13912

[sapphi-red]

• feat(minifier): inline single use variables within the same variable declarations #13959 👈 (View in Graphite)
• main

---

How to use the Graphite Merge Queue

Add either label to this PR to merge it via the merge queue:

• 0-merge - adds this PR to the back of the merge queue
• hotfix - for urgent hot fixes, skip the queue and merge this PR next

You must have a Graphite account in order to use the merge queue. Sign up using this link.

An organization admin has enabled the Graphite Merge Queue in this repository.

Please do not merge from GitHub as this will restart CI on PRs being processed by the merge queue.

This stack of pull requests is managed by Graphite. Learn more about stacking.

[codspeed-hq]

CodSpeed Instrumentation Performance Report

Merging #13959 will not alter performance

_{Comparing 09-20-feat_minifier_inline_single_use_variables_within_the_same_variable_declarations (c0cb248) with main (6fcb0d0)}

Summary

✅ 37 untouched

[Copilot]

Pull Request Overview

Adds a peephole optimization to inline single-use variables within the same variable declaration list (e.g., var a = foo, b = a; -> var b = foo;), and refactors internal logic to support reuse across contexts.

• Implement inline-within-declaration optimization and invoke it for standalone var/let/const declarations and for-loop init declarations
• Extract shared logic for substituting single-use symbols from a list of declarators
• Update tests and snapshots to reflect improved minification and allocation/sizing deltas

Reviewed Changes

Copilot reviewed 5 out of 5 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
tasks/track_memory_allocations/allocs_minifier.snap	Updates memory allocation snapshots reflecting new optimization effects.
tasks/minsize/minsize.snap	Updates size metrics snapshots showing slightly improved minification.
crates/oxc_minifier/tests/peephole/inline_single_use_variable.rs	Adds tests covering inlining within the same variable declarations, including for-loop init and using/let cases.
crates/oxc_minifier/src/peephole/minimize_statements.rs	Implements the inlining within declaration lists, refactors substitution logic into a reusable helper, and wires calls at appropriate transformation points.
crates/oxc_minifier/src/peephole/minimize_conditions.rs	Adjusts a test to expect additional constant folding/elimination due to the new optimization.

---

_{Tip: Customize your code reviews with copilot-instructions.md. Create the file or learn how to get started.}